1. Technical Field
The present invention relates to an organic light emitting diode display, and more particularly, to an organic light emitting diode display and its driving method, in which three or more transistors are provided in each pixel.
2. Discussion of the Related Art
Examples of flat panel displays (FPDs) which has been recently used widely include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence device.
Since the PDP is simple in its structure and manufacturing process, it has received attention as a display which is lightweight, thin, short and small, and is the most advantageous for a large sized screen. However, the PDP has a problem in that it has low light emitting efficiency and luminance and has high power consumption. Although a thin film transistor (TFT) LCD to which a TFT is applied as a switching device is the flat panel display device which is used most widely, since it is a non-light emitting device, it has a problem in that a viewing angle is narrow and a response speed is low.
By contrast, the electroluminescence device is classified into an inorganic light emitting diode display and an organic light emitting diode display depending on a material of a light emitting layer. Particularly, the organic light emitting diode display has advantages in that a response speed is fast, and the light emitting efficiency, luminance and viewing angle are improved compared to the other types of FPDs described above by using a self-light emitting device that light emits by itself.
FIG. 1 is a circuit diagram illustrating a structure of one pixel of an organic light emitting diode display according to the related art, specifically illustrating a pixel structure of two N type transistors. FIG. 2 is an exemplary view illustrating various waveforms applied to an organic light emitting diode display according to the related art, especially illustrating waveforms applied to an organic light emitting diode display that requires four gate signals for one pixel.
As shown in FIG. 1, a pixel 50 of the organic light emitting diode display according to the related art may include an organic light emitting diode (OLED) and at least two transistors T1 and T2 connected to a data line DL and a gate line Gn to control the organic light emitting diode (OLED).
An anode electrode of the organic light emitting diode is connected to a first power source VDD, and its cathode electrode is connected to a second power source VSS. The organic light emitting diode generates light of predetermined luminance in response to a current supplied from the second transistor T2.
Various circuits formed in the pixel 50 control the amount of current supplied to the organic light emitting diode in response to an image signal supplied to a data line DL when a scan signal is supplied to the gate line Gn. To this end, the pixel 50 includes the second transistor T2 (driving transistor) connected between the first power source VDD and the organic light emitting diode, the first transistor T1 (switching transistor) connected among the second transistor T2, the data line DL and the gate line Gn, and a storage capacitor Cst connected between a gate electrode of the second transistor T2 and the organic light emitting diode.
In the meantime, although only one gate signal (scan signal) may be input to the organic light emitting diode display shown in FIG. 1, the organic light emitting diode display generally uses two or more gate signals.
In other words, each pixel 50 of the aforementioned organic light emitting diode display needs a compensation circuit to remove luminance non-uniformity, that is, Mura defects, as well as the switching transistor T1 and the driving transistor T2. Accordingly, a plurality of gate signals are required to control a plurality of transistors applied to the compensation circuit. Examples of the gate signals may include various kinds of signals such as an emission signal for controlling an emission transistor in addition to the scan signal for controlling the switching transistor that supplies an image signal (data voltage) transmitted through the data line to the pixel.
Accordingly, the organic light emitting diode display according to the related art may include three or more transistors in one pixel, or may include four or more transistors.
In other words, although one gate signal (scan signal) is only transmitted to one pixel in a liquid crystal display, at least two gate signals including the scan signal should be transmitted to one pixel in the organic light emitting diode display, whereby the pixel may be driven normally.
Particularly, in case of a gate driver of the organic light emitting diode display in which four gate signals should be applied to one pixel, as shown in FIG. 2, a gate output enable signal GOE is fixed at low level L, and each gate signal is synchronized at one clock (gate shift clock (GSC)), whereby four gate signals are respectively output at the time of rising of the clock.
However, since the organic light emitting diode display of the related art has no option whether to output a gate signal at the time of rising of the gate shift clock GSC or at the time of falling of the gate shift clock GSC, transistors adjacent to one another within one pixel are operated in a unit of 1 period of the clock CLK. In other words, as shown in FIG. 2, supposing that the period between the rising time of the gate shift clock GSC and next rising time is 1 period, in the organic light emitting diode display of the related art, a first gate signal X1 input to the first transistor and a fourth gate signal X4 input to the fourth transistor are output to the pixel at the time of rising of the gate shift clock, and a second gate signal X2 input to the second transistor and a third gate signal X3 input to the third transistor are output to the pixel at the time of next rising of the gate shift clock. The gate signals X5, X8, X6, and X7 describe gate signals input to transistors of another pixel.
In other words, the respective gate signals are input to the pixel at an interval as much as 1 period (1 clock) of the gate shift clock.
As described above, in the organic light emitting diode display of the related art, since the gate signals are input to one pixel at an interval as much as 1 period of the gate shift clock, the time for driving all the transistors formed in one pixel is increased. For this reason, a problem occurs in that the time when an image is output is delayed.